As part of our ongoing investigations involving lectin-mediated adhesion among oral bacteria, the receptor polysaccharide from Streptococcus gordonii 38 was isolated and characterized. Carbohydrate analysis of the hydrolysed S. gordonii 38 polysaccharide by high-performance anion-exchange chromatography with pulsed amperometric detection (HPAEC-PAD) showed galactose (Gal) (2 mol), N-acetylgalactosamine (GalNAc) (1 mol), rhamnose (Rha) (2 mol), glucose (Glc) (1 mol) and galactosamine-6-phosphate (1 mol). Mild acid hydrolysis of the polysaccharide yielded a heptasaccharide repeating unit. The structure of the heptasaccharide repeating unit was determined by high-resolution NMR spectroscopy which includes various homonuclear (DQF-COSY, TQF-COSY, NOESY and HOHAHA) and heteronuclear experiments (HMQC), including linkage assignments by 1H-13C long-range correlation (HMBC). Complete 1H and 13C NMR assignments for the intact polysaccharide yielded the covalent structure of a heptasaccharide repeating unit: [Formula: see text] The structure of the strain 38 polysaccharide is closely related to those of Streptococcus mitis J22 and Streptococcus oralis 34. Thus, the difference between the strain 38 and J22 heptasaccharides was at their reducing ends, with GaLNAc beta-(1-->3)-Gal in the former and Gal beta-(1-->3)-GalNAc in the latter, while the difference between the 38 heptasaccharide and 34 hexasaccharide was at the non-reducing ends, where a rhamnose branch occurred in the former but not the latter structure. When compared by their quantitative precipitin curves with rabbit antibodies against each streptococcal strain, the strain 38 polysaccharide reacted more like the polysaccharide of strain J22 than that of strain 34. In contrast, each strain was recognized by the Gal- and GalNAc-reactive lectins of Actinomyces spp., but only strains 38 and 34 were recognized by GalNAc-sensitive lectins of other streptococci. These findings strongly support the hypothesis that the immunogenic features of these polysaccharides are distinct from those detected by lectin binding.